1. Field of the Invention
The present invention relates to the compression and expansion process of an image data using the discrete cosine transform technique, in particular, relative to the reduction of the mosquito noise.
2. Description of Related Art
The JPEG (Joint Photographic Experts Group) method based on the discrete cosine transform is widely used as a means of compressing and expanding the image data. Image processing devices such as copying machines, scanners and printers apply the compression and expansion process to the image data in order to reduce the capacity requirement of the memory used for storing the image data. However, the compression and expansion process using the discrete cosine transform tends to generate mosquito noises which deteriorate the image quality in the vicinity of edges where the density values of pixels change sharply.
Publication of Unexamined Japanese Patent Application JP-A-5-294018 discloses a technique of removing mosquito noises. The technique removes the noises by, after compressing an image data, correcting an image data in an edge region during the expansion of the compressed image data.
The technology disclosed in the above publication is a technology of allowing the generation of the mosquito noises during the compression of the image data containing the edge region using the JPEG method, and removing the noises by another process later.
However, if the mosquito noises are too large, the process of correcting the image data in the edge region by treating the image data with the density value below a specified value as noises may inadvertently delete a part of the original image together with the mosquito noises. Therefore, it is necessary to reduce the generation of the mosquito noises themselves in order to remove the mosquito noises without losing part of the original image.